Process for preparing a supported silver catalyst

ABSTRACT

This invention relates to a supported silver catalyst for the manufacture of ethylene oxide prepared by a process comprising impregnating a porous catalyst support with a solvent containing a silver salt and treating the impregnated support to effect deposition of silver on the support surface. Following silver deposition, the support is impregnated with a liquid containing more than 30%, by volume, of an organic solvent capable of forming a complex with silver ion, and a compound of at least one metal promoter in an amount sufficient to deposit the desired amount of metal cation on said support. The impregnated support is then treated to effect deposition of the promoter. There is also described herein a process of making such catalyst and a process for producing ethylene oxide.

This invention relates to supported silver catalysts for the manufactureof ethylene oxide, their preparation, and their use in ethylene oxideprocesses. More specifically, the invention is concerned with preparinga metal cation promoted silver catalyst capable of oxidizing ethylenewith an oxygen-containing gas in the vapor phase to produce ethyleneoxide at high efficiencies.

In characterizing catalysts useful for the manufacture of ethyleneoxide, the term "selectivity" is employed herein as defined in U.S. Pat.No. 3,420,784, patented Jan. 7, 1969, at column 3. The terms"efficiency" and "selectivity" as used throughout the specification withregard to the aforesaid catalysts are intended to be synonymous.

Processes for preparing metal cation-promoted silver catalysts for theproduction of ethylene oxide are extensively described in the patentliterature. The vast majority of these processes employ impregnationtechniques wherein solutions containing solubilized compounds of silverand metal cation promoters are used to impregnate a porous carrier orsupport followed by heat treatment of the impregnated support to effectdeposition of the silver and metal cation on the support. Processes formaking coated catalysts employ techniques wherein silver and metalcations are coated onto a catalyst support from an emulsion or slurryfollowed by a heating step to remove the liquid present from the carrierand effect deposition of the silver and metal promoter. Coated catalystsare generally considered today to be less satisfactory than impregnatedcatalysts in commercial practice because it is generally believed thatcoating methods are unable to accomplish substantial deposition ofsilver into the interior surfaces of the carrier and consequently, thecoated catalysts are more susceptible to silver loss by mechanicalabrasion.

The impregnation methods described in the art for preparing ethyleneoxide catalysts include a wide variety of methods of depositing silverand metal cations onto a carrier. These methods are generallydistinguished by the process conditions they employ such aslow-temperature impregnation, high temperature impregnation, activationin an inert gas atmosphere and/or choice of solvent for the silverimpregnating solution.

Criticality is often taught to reside in the order of addition of themetal cation and silver to the carrier. Such processes are characterizedby their employing either a coincidental (or simultaneous) method ofdepositing silver and metal cation onto the carrier or a sequentialmethod of addition wherein silver is added either before or after themetal cation. The addition of silver to a carrier subsequent to theaddition of metal cation is referred to herein as a "metal-first"sequential process of preparation, while the addition of silver to thecarrier prior to the addition of the metal cation is referred to hereinas a "silver-first" method of preparation. The coincidental (orsimultaneous) addition of silver and metal cation to a carrier isreferred to herein as a "coincidental method" of preparation. The use ofthe term "addition" of a metal cation and/or silver to a carrier ismeant to include the steps of impregnating the porous carrier with asolution containing silver and/or metal cation, as the case may be,followed by deposition of same upon the carrier, usually by heattreatment.

The comparative performance of catalysts produced by coincidental andsequential methods of impregnation has been reported in the art. Forexample, U.S. Pat. No. 3,563,914 to Wattimena, in Table III, comparesthe effect of the order of addition of alkali metal promoter and silverto a catalyst support on catalyst efficiency. The data in Table III issaid to illustrate the advantage of adding an alkali metal promoter tothe support before the silver compound. Specifically, the catalystsprepared by an alkali metal-first preparation procedure are shown tohave an efficiency of 4-5 percent higher than catalysts prepared by acoincidental deposition of alkali metal and silver. Further, a catalystprepared by the addition of silver to the carrier prior to alkali metaladdition was by far the least efficient in that the selectivity wasabout 12 percent below that of a similar catalyst prepared by acoincidental method of deposition. In contrast with Wattimena'sconclusion regarding the superiority of an alkali metal-first sequentialorder of addition, Belgian Pat. No. 793,658 and U.S. Pat. Nos.3,962,136, 4,101,115 and 4,012,425 to Nielsen et al indicate that thecoincidental deposition of silver and alkali metal is the preferredmethod of catalyst preparation insofar as it results in the highestcatalyst efficiencies. The aforementioned Belgian patent also provides adirect comparison of catalysts prepared by a method of coincidentaldeposition of silver and potassium with catalysts of similar compositionprepared by a sequential process wherein silver is deposited prior topotassium. Specifically, Example III of the Belgian patent indicatesthat the maximum efficiency achieved with catalysts containing 7.8weight percent silver and varying amounts of co-deposited potassium was76.3% under the stated test conditions whereas in Example VII of thepatent the maximum selectivity achieved under the same conditions withcatalysts containing the same amount of silver and similar amounts ofpotassium but prepared by a silver-first process was 73-74%, thusconfirming the data in Wattimena concerning the inherent inefficiency ofcatalysts prepared by a silver-first sequential order of addition.

U.S. Pat. No. 4,207,210 to Kilty, based upon British specification No. 1489 335, describes an alkali metal-first process for preparing ethyleneoxide catalysts which is said to provide catalysts equivalent or evensuperior to those produced by coincidental methods of deposition such asset forth in the aforementioned U.S. patents to Nielsen et al. Accordingto the described procedure of Kilty, an aqueous solution containingalkali metal is used to impregnate the porous carrier which is thendried to fix the alkali metal and thereafter the silver is supplied tothe support. Tables A through E of the Kilty U.S. patent providecomparisons of catalysts prepared in accordance with the disclosedalkali metal-first method of addition with catalysts of similarcomposition prepared by the simultaneous addition of alkali metal andsilver. The criticality of the alkali metal-first method of addition is,however, called into question by the reported data which fails toindicate any discernible difference between either method of preparationbased on the measured catalyst efficiencies. Indeed, the alkalimetal-first method of addition appears to be inherently identical to thecoincidental deposition method used in the Nielsen et al patents asevidenced by the fact that both Kilty and Nielsen et al disclose thatthe alkali metal which is added to the carrier can be subsequentlyremoved, if desired, using an alkanol solvent. This suggests that in thepreparation procedure of Kilty, the alkali metal which is initiallydeposited on the carrier is resolubilized in the silver-containingimpregnating solution thereby inherently effecting a coincidentaldeposition of silver and alkali metal. (Compare this to Wattimena, U.S.Pat. No. 3,563,914, discussed above.) This is further evidenced by acomparison of the curve shown in Kilty's British Patent SpecificationNo. 1 489 335 wherein selectivity is plotted as a function of cesiumcontent for a carrier having a surface area of 0.19 m² /g, and the curveshown in Ser. No. 216,188, filed Jan. 7, 1972, now abandoned (theapplication from which the Nielsen et al U.S. patents were derived),wherein curve C represents as a function of cesium content theselectivities achieved with catalysts having an essentially similarsilver content and alumina carrier to that used in the examples of Kiltybut prepared by a coincidental method of deposition. The similarity ofthe two curves confirms the fact that the efficiencies produced withcatalysts prepared by the coincidental method of Nielsen et al and thesequential method of Kilty are essentially equivalent.

As noted in the prior art, processes for preparing catalysts by thesilver-first method have obvious drawbacks with regard to the resultingcatalyst efficiencies. The prior art has documented the markedly lowerefficiencies of catalysts produced by the silver-first method relativeto similar catalysts prepared by a coincidental method, the latterappearing to be essentially equivalent to an alkali metal-first order ofaddition. Thus, as discussed above, U.S. Pat. No. 3,563,914 to Wattimenaand Belgian Pat. No. 793,658 contain comparative data clearlyillustrating the relative inefficiency of catalysts produced by asilver-first sequential method of addition relative to a coincidentalmethod of addition. While other patents in the art directed tosilver-first methods of preparation do not provide sufficient data toallow such side-by-side comparisons to be made, nevertheless, the datawhich is provided appears to indicate that silver-first methods are theless preferred methods. U.S. Pat. No. 4,033,903 to Maxwell, for example,discloses a silver-first method of addition wherein used ethylene oxidecatalysts are reactivated by the addition of an alkali metal promoter tothe aged catalyst. The process of the patent is said to be equallyeffective for enhancing the efficiency of freshly prepared catalysts byemploying a heat treatment step intermediate to the steps of silveraddition and alkali metal addition to the carrier. The effectiveness ofthis method of preparation seems somewhat doubtful, however, in view ofthe data shown in Table III of the patent wherein catalysts R and T,catalysts prepared by a silver-first method are shown to be inferior tocatalyst Q, a silver catalyst containing no alkali metal promoter.Accordingly, based upon the data in the aforementioned patents thereappears to be an obvious need in the art for a silver-first sequentialmethod of catalyst preparation capable of providing catalysts which areno less efficient than those produced by the coincidental or metal-firstmethods.

A common characteristic of the various silver-first methods ofpreparation described in the literature is their use of the samesolvents for metal cation addition. That is, the methods disclosed inthis literature suggest using water or a lower alcohol, such as,methanol or ethanol, as the solvent for effecting metal cationimpregnation. Thus, for example, the aforementioned patent to Wattimenadescribes in Example III a silver-first addition wherein water isemployed as the solvent for the alkali metal impregnation step. BelgianPat. No. 793,658 which discloses a silver-first method of addition inExample VII thereof states that aqueous solutions of potassium were usedas the impregnating medium for the promoter. U.S. Pat. No. 4,066,575 toWinnick describes a process of catalyst preparation characterized by anactivation step wherein the carrier is heated in an inert gas atmospherefollowing its impregnation with a silver solution. An alkali metalpromoter is thereafter deposited on the carrier employing as a solventfor the alkali metal, water or a lower alkanol such as, methanol,ethanol or propanol. Great Britain Patent Application No. 2,045,636Aattempts to distinguish itself from the prior art processes by itslow-temperature deposition technique whereby the carrier impregnatedwith a silver-containing solution is maintained at temperatures below200° C. prior to the so-called post deposition of alkali metal. Thesuggested solvents for such post-deposition of alkali metal are waterand ethanol. German Offenlegungsschrift No. 2,914,640 discloses asequential order of impregnation wherein silver is initially applied tothe carrier from a suspension and the carrier thereafter immediatelydried. Alkali metal is then added to the carrier from a solution usingwater as the solvent. U.S. Pat. No. 4,248,740 to Mitsuhata et aldescribes a catalyst preparation procedure employing a silver-firstorder of addition. The patentees recommend impregnating the carrier withan alkali metal solution containing water or a lower alcohol, such asmethanol, ethanol or propanol. The solvent is then evaporated, carebeing taken to prevent heating of the catalyst to above 200° C., acritical feature of the described process. In U.S. Pat. No. 4,168,247 toHayden et al, there is described a preparation procedure for catalystsidentified by the numbers 34-37 which consists of a silver-first orderof addition. The alkali metal promoters were dissolved in water withfurther addition of methanol, and the resulting solution used toimpregnate the carrier.

Japanese Patent Application No. 142,421/78 (Kokai No. 79,193/79)discloses a "post-treatment" of a used or stabilized silver catalyst byimpregnating such catalyst with a solution containing an alkali metalpromoter, an organic compound capable of forming a complex salt withsilver ion and an alcohol of 1 to 4 carbon atoms. No alcohol other thanmethanol was used in the impregnating solution described in theexamples. A further distinction between the process of the reference andthe present invention resides in the fact that the improved efficienciesachieved in the examples of the reference can be attributable to thepresence of an oxide of nitrogen in the catalyst (see, for example, GBNo. 2,014,133A which discloses the beneficial effects of nitrates ornitrite forming substances in the manufacture of ethylene oxide), ratherthan, the promoting effect of alkali metal in accordance with thepresent invention.

SUMMARY OF THE INVENTION

The invention describes a process for preparing a supported silvercatalyst for the production of ethylene oxide by the vapor phaseoxidation of ethylene with an oxygen-containing gas, the catalystproduced by such process and the use of such silver catalyst forethylene oxide manufacture. The process comprises impregnating a porouscatalyst support with a solution comprising a solvent or a solubilizingagent, and silver salt in an amount sufficient to deposit the desiredamount of silver on said support. The impregnated support is thentreated to convert at least a fraction of the silver salt to silvermetal and effect deposition of silver on the surface of said support.Following silver deposition, the support is impregnated with a liquidcontaining more than 30%, by volume, of an organic solvent capable offorming a complex with silver ion, and a compound of at least one metalcation promoter in an amount sufficient to deposit the desired amount ofmetal cation on said support. In accordance with another embodiment ofthe invention the impregnating solution containing the metal promotersis kept substantially free of lower alcohols, which as used hereinrefers to alcohols containing from 1 to 4 carbon atoms. The impregnatedsupport is thereafter treated to effect deposition of the promoter onthe surface of said support.

The catalyst preparation process of the invention, in its broadestaspect, concerns a process wherein silver and a metal promoter aresequentially deposited on the surfaces of a porous carrier by asilver-first method. The particular metal promoter employed is notcritical to the invention and may include one or more alkali metals,such as lithium, sodium, potassium, rubidium and/or cesium; one or morealkaline earth metals, such as, barium, magnesium and strontium; or oneor more of the other known promoters, such as thallium, gold, tin,antimony and rare earths; and the like. For purposes of convenience, thecatalyst preparation process of the invention is described below interms of a silver-first method of preparation wherein the promoter isselected from among alkali metals, it being recognized that otherpromoters of silver catalysts, such as those mentioned above, mayoptionally be substituted for or added to alkali metals in such process.

The process of the invention is predicated on the discovery that acatalyst preparation procedure employing a silver-first addition ofsilver and metal cation to a porous carrier can provide catalysts asefficient as those produced by the coincidental deposition of the sameonto the same or similiar carrier provided that the solvent for themetal cation impregnating solution is selected in accordance with theinvention. That is, contrary to prior art experiences with silver-firstmethods of preparation wherein the resulting catalysts invariably areless efficient, even at their optimum, than corresponding catalystsprepared by a method of coincidental deposition, the catalysts of theinvention provide improved selectivities to ethylene oxide and areequally as efficient as catalysts produced by coincidental methods ofpreparation.

As used herein with reference to the silver catalyst and process of theinvention, the term "optimum" efficiency is defined as the highestefficiency obtainable at any concentration of promoter for a givensilver content, catalyst carrier, and method of preparation when testedat fixed operating conditions.

The solvent employed in the metal cation impregnating solution is anessential feature of the present invention. The organic solvents usefulfor the invention are characterized by their capability of forming asilver complex in the presence of silver ion. Contrary to the disclosurein the aforementioned Japanese Patent Application No. 142,421/78, it hasbeen found that such silver complex forming solvents can be effectivelyused in a silver-first sequential process of preparation in amounts inexcess of 30%, by volume, of the impregnating solution. Indeed, ashereinafter described, depending upon the solubility of the metalpromoter in such solvents, they are advantageously employed in theimpregnating solution in concentrations as high as possible, generallyabove 50 weight %, and preferably about 80% or higher by weight ofsolution. Further, unlike the process of the aforementioned JapaneseApplication, the presence of a lower alcohol in the impregnatingsolution is not required in the method of the present invention. Theterm "lower alcohol" as used in this specification and the claims meansan alcohol of not more than four carbon atoms.

Suitable solvents used for impregnating the metal promoter in accordancewith the invention include, among others, amino alcohols such asmonoethanolamine; alkylene diamines such as ethylenediamine; alkylamines such as isopropylamine; amino ethers such as bis(2-amino) ethylether; and amides such as formamide.

In addition to the aforementioned improved catalyst efficiencies,another important characteristic of the process of the invention and onewhich provides an unexpected advantage over conventional methods ofcatalyst preparation relates to the fact that the amount of alkali metalpromoter deposited upon the carrier need not be as narrowly controlledas in the prior art to achieve an optimum catalyst efficiency. It isknown in the art that the coincidental method of producing ethyleneoxide catalysts requires strict control of the amount of promoter addedto the carrier in order to maximize the catalyst efficiency for thegiven carrier and silver content. The effect of promoter concentrationon catalyst efficiency is graphically demonstrated by the drawingpresented in the above-mentioned U.S. Ser. No. 216,188 (the parentapplication of the Nielsen et al U.S. patent ) which depicts therelative effects of cesium, rubidium and potassium as respectivepromoters in enhancing the efficiency of a silver catalyst to makeethylene oxide. Curves A, B and C of the drawing show the appropriateconcentration ranges in which potassium, rubidium and cesium,respectively, provide the greatest degree of selectivity enhancement.From the curves it is evident that the amount of alkali metal which mustbe added to the carrier is critical if the maximum catalyst efficiencyis to be realized. By way of comparison, in the present process thepromoter concentration required to produce catalysts having optimumselectivities to ethylene oxide is not as narrowly critical. Forexample, the range of alkali metal concentrations capable of providingthe optimum efficiency is far broader than the corresponding range forcatalysts produced by coincidental methods of preparation in whichalkali metals are the promoters. Thus, an important advantage of thepresent process resides in the fact that commercial-scale batches ofethylene oxide catalysts can be manufactured within a relatively broadspecification of the metal content and still achieve optimum efficiency.

When alkali metals are the promoters, the amount of alkali metal neededon the catalyst support according to the process of this invention toachieve an optimum efficiency is typically at least 10% greater thanthat amount of like alkali metal which provides the maximum enhancementof efficiency when used in a coincidental method of preparation with thesame amount of silver and the same catalyst support. Even though this isthe case, the amount of alkali metal to achieve optimum efficiency isstill not as narrowly critical and will vary depending upon silvercontent, the catalyst support employed, the solvent for the alkali metalimpregnating solution, and other catalyst preparation variables.

CATALYST PREPARATION

The catalyst preparation method of the invention concerns a silver-firstsequential addition of silver and metal cation promoter to a porouscarrier. Stated simply, the process involves a sequence of steps carriedout in the following order:

First, impregnating a porous catalyst support by immersing same in asilver-containing impregnating solution;

Second, treating the impregnated support to effect deposition of silveron the surface of said support;

Third, impregnating the product of step two by immersing same in a metalcation-containing impregnating solution as defined herein; and

Fourth, treating the impregnated support to effect deposition of themetal promoter on the surface of said support.

Silver deposition is generally accomplished by heating the impregnatedcarrier at elevated temperatures to evaporate the liquid within thesupport and effect deposition of the silver onto the interior andexterior carrier surfaces. Alternatively, a coating of silver may beformed on the carrier from an emulsion or slurry containing the samefollowed by heating the carrier as described above. Impregnation of thecarrier is generally the preferred technique for silver depositionbecause it utilizes silver more efficiently than coating procedures, thelatter being generally unable to effect substantial silver depositiononto the interior surfaces of the carrier.

The silver solution used to impregnate the carrier is comprised of asilver salt or compound in a solvent or complexing/solubilizing agentsuch as the silver solutions disclosed in the art. The particular silversalt employed is not critical and may be chosen, for example, from amongsilver nitrate, silver oxide or silver carboxylates, such as, silveracetate, oxalate, citrate, phthalate, lactate, propionate, butyrate andhigher fatty acid salts.

A wide variety of solvents or complexing/solubilizing agents may beemployed to solubilize silver to the desired concentation in theimpregnating medium. Generally, the silver concentration in theimpregnating medium should be sufficient to deposit on the support fromabout 2 to about 20 wt. % of silver based on the total weight of thecatalyst. Among solvents disclosed in the art as being suitable for thispurpose are lactic acid (U.S. Pat. Nos. 2,477,435 to Aries; and3,501,417 to DeMaio); ammonia (U.S. Pat. No. 2,463,228 to West et al);alcohols, such as ethylene glycol (U.S. Pat. Nos. 2,825,701 to Endler etal; and 3,563,914 to Wattimena); and amines and aqueous mixtures ofamines (U.S. Pat. Nos. 2,459,896 to Schwartz; 3,563,914 to Wattimena,3,702,259 to Nielsen; and 4,097,414 to Cavitt).

Following impregnation of the catalyst carrier with silver, theimpregnated carrier particles are separated from any remainingnon-absorbed solution or slurry. This is conveniently accomplished bydraining the excess impregnating medium or alternatively by usingseparation techniques, such as, filtration or centrifugation. Theimpregnated carrier is then generally heat treated (e.g., roasted) toeffect decomposition and reduction of the silver metal salt to metallicsilver. Such roasting may be carried out at a temperature of from about100° C. to 900° C., preferably from 200° C. to 700° C., for a period oftime sufficient to convert substantially all of the silver salt tosilver metal. In general, the higher the temperature, the shorter therequired reduction period. For example, at a temperature of from about400° C. to 900° C., reduction may be accomplished in about 1 to 5minutes. Although a wide range of heating periods have been suggested inthe art to thermally treat the impregnated support, (e.g., U.S. Pat. No.3,563,914 suggests heating for less than 300 seconds to dry but notroast reduce the catalyst; U.S. Pat. No. 3,702,259 discloses heatingfrom 2 to 8 hours at a temperature of from 100° C. to 375° C. to reducethe silver salt in the catalyst; and U.S. Pat. No. 3,962,136 suggests1/2 to 8 hours for the same temperature range) it is only important thatthe reduction time be correlated with temperature such thatsubstantially complete reduction of the silver salt to metal isaccomplished. A continuous or step-wise heating program may be used forthis purpose.

Impregnation of the carrier with a solution containing a promoter saltor compound is carried out after silver deposition has been effected.The impregnating solution is prepared using one or more solvents asherein defined and contains an amount of promoter sufficient to achievethe desired concentration of promoter in the finished catalyst. Theimpregnated carrier particles are conveniently separated from anyremaining non-absorbed solution by draining the excess impregnatingsolution or alternatively by using separation techniques, such as,filtration and centrifugation. The impregnated carrier is then generallyheat treated at ambient or sub-atmospheric pressure to remove thesolvent (or solvents) present and deposit (with or withoutdecomposition) the alkali metal ions on to the silver and carriersurfaces. Such heating may be carried out at a temperature of from about50° C. to 900° C., preferably from about 100° C. to 700° C. and mostpreferably from about 200° C. to about 600° C.

Suitable alkali metal promoter compounds include all those soluble inthe particular solvent or solubilizing agent employed. Accordingly,inorganic and organic compounds of alkali metals, such as, nitrates,halides, hydroxides, sulfates and carboxylates may be used. An inherentadvantage of the process of the invention is that it allows the use ofcertain promoter compounds which could not ordinarily be used inconjunction with known coincidental methods of preparation because ofthe incompatibility of such salts with the impregnating solution used inthe latter processes. As an illustration, alkaline earth salts such assalts of barium, calcium and magnesium can readily be solubilized in animpregnating solution and deposited upon the carrier in accordance withthe process of the invention, but can not be added to an impregnatingsolution containing, for example, oxalic acid or carboxylic acid,solutions commonly employed in conventional coincidental methods ofpreparation for purposes of silver solubilization.

The types of solvents useful for preparing the promoter impregnatingsolution are set forth above. Such solvents may be employed individuallyor in various combinations with each other provided that the salt of thedesired promoter is sufficiently soluble therein. In the event that thepromoter salt is not sufficiently soluble in the organic solvent toprovide the desired concentration in the resulting impregnatingsolution, water may be added as a co-solvent for the promoter salt.Generally, organic solvent concentrations of 50 wt. % and higher arecommonly employed for impregnation. In general, it is preferred that theconcentration of organic solvent in the impregnating solution be as highas possible.

Heat treatment of the impregnated carriers is preferably carried out inair, but a nitrogen, carbon dioxide or hydrogen atmosphere may also beemployed. The equipment used for such heat treatment may use a static orflowing atmosphere of such gases to effect reduction.

The particle size of silver metal deposited upon the carrier is afunction of the catalyst preparation procedure employed. Thus, theparticular choice of solvent and/or complexing agent, silver salt, heattreatment conditions and catalyst carrier may affect, to varyingdegrees, the size of the resulting silver particles. For carriers ofgeneral interest for the production of ethylene oxide, a distribution ofsilver particle sizes in the range of 0.05 to 2.0 microns is typicallyobtained.

CARRIER SELECTION

The catalyst carrier employed in practicing the invention may beselected from conventional, porous, refractory materials which areessentially inert to ethylene, ethylene oxide and other reactants andproducts at reaction conditions. These materials are generally labelledas "macroporous" and consist of porous materials having surface areasless than 10 m² /g (square meters per gram of carrier) and preferablyless than 1 m² /g. The surface area is measured by the conventionalB.E.T. method described by Brunauer, S., Emmet, P., and Teller, E., inJ. Am. Chem. Soc. Vol. 60, pp 309-316, (1928). They typically possesspore volumes in the range of about 0.15-0.8 cc/g. A more preferred rangeis about 0.2-0.6 cc/g. Pore volumes may be measured by conventionalmercury porosimetry or water absorption techniques. Median porediameters for the above-described carriers range from about 0.01 to 100microns, a more preferred range being from about 0.5 to 50 microns.

Preferably, the carrier should not contain ions which are exchangeablewith the metal cations supplied to the catalyst, either in thepreparation or use of the catalyst. If the carrier contains such ion,the ion should be removed by standard chemical techniques such asleaching.

The chemical composition of the carrier is not narrowly critical.Carriers may contain fused or bonded particles of, for example, ofalpha-alumina, silicon carbide, silicon dioxide, zirconias, magnesia andvarious clays. In general, alpha-alumina based materials are preferred.These alpha-alumina based materials may be of very high purity, i.e.,98+weight % alpha-alumina, the remaining components being silica, alkalimetal oxides (e.g., sodium oxide) and trace amounts of other metal andnon-metal impurities; or they may be of lower purity, i.e., about 80weight % alpha-alumina, the balance being a mixture of silicon dioxide,various alkali oxides, alkaline earth oxides, iron oxide, and othermetal and non-metal oxides. The lower purity carriers are formulated soas to be inert under catalyst preparation and reaction conditions. Awide variety of such carriers are commercially available. The carriersare preferably shaped, typically in the form of pellets, extrudedparticles, spheres, rings and the like, for use in commercial reactors.The size of the carriers may vary from about 1/16" to 1/1". The carriersize and shape is chosen to be consistent with the type of reactoremployed. In general, for fixed bed reactor applications, sizes in therange of 1/8" to 3/8" have been found to be most suitable in the typicaltubular reactor used in commercial operations.

ETHYLENE OXIDE PRODUCTION

The silver catalysts of the invention are particularly suitable for usein the production of ethylene oxide by the vapor phase oxidation ofethylene with molecular oxygen. The products of the reactions areethylene oxide and CO₂ as a consequence of the following two competingreactions:

    C.sub.2 H.sub.4 +1/2O.sub.2 →C.sub.2 H.sub.4 O      (1)

    C.sub.2 H.sub.4 +3O.sub.2 →2CO.sub.2 +2H.sub.2 O    (2)

The success in making reaction (1) more favored results in higherprocess efficiencies to ethylene oxide. The reaction conditions forcarrying out the oxidation reaction are wellknown and extensivelydescribed in the literature. This applies to reaction conditions, suchas, temperature, pressure, residence time, concentration of reactants,diluents (e.g., nitrogen, methane and recycled CO₂), inhibitors (e.g.,ethylene dichloride) and the like. In addition, the desirability ofrecycling unreacted feed, or employing a single-pass system, or usingsuccessive reactions to increase ethylene conversion by employingreactors in series arrangement can be readily determined by thoseskilled in the art. The particular mode of operation selected willusually be dictated by process economics.

Generally, the process is carried out by continuously introducing a feedstream containing ethylene and oxygen to a catalyst-containing reactorat a temperature of from about 200° to 300° C., and a pressure which mayvary from one atmosphere to about 30 atmospheres depending upon the massvelocity and productivity desired. Residence times in large-scalereactors are generally on the order of about 1-5 seconds. Oxygen may besupplied to the reaction in an oxygen-containing stream, such as, air oras commercial oxygen. The resulting ethylene oxide is separated andrecovered from the reaction products using conventional methods.Byproduct CO₂ is usually recycled in part with the unreacted ethylene tothe reaction in commercial operations.

CATALYST TESTING

The catalysts cited in the Tables of the Examples below were allevaluated under standard test conditions using backmixed,bottom-agitated "magnedrive" autoclaves as described in FIG. 2 of thepaper by J.M. Berty entitled "Reactor For Vapor Phase-CatalyticStudies", in Chemical Engineering Progress, Vol. 70, No. 5, pages 78-84,1974. The reactor was operated at 1.0 mole % ethylene oxide in theoutlet gas under the following standard inlet conditions:

    ______________________________________                                        Component       Mole %                                                        ______________________________________                                        Oxygen          6.0                                                           Ethylene        8.0                                                           Ethane           0.50                                                         Carbon Dioxide  6.5                                                           Nitrogen        Balance of Gas                                                Parts per millon                                                                              7.5                                                           Ethylene Chloride                                                             ______________________________________                                    

The pressure was maintained constant at 275 psig and the total outletflow maintained at 22.6 SCFH..sup.(1) The outlet ethylene oxideconcentration was maintained at 1.0% by adjusting the reactiontemperature. Thus, temperature (°C.) and catalyst efficiency areobtained as the responses describing the catalyst performance.

A typical catalyst test procedure is comprised of the following steps:

1. 80 cc of catalyst is charged to a backmixed autoclave. The volume ofcatalyst is measured in a 1" I.D. graduated cylinder after tapping thecylinder several times to thoroughly pack the catalyst. The weight ofthe catalyst is noted.

2. The backmixed autoclave is heated to about reaction temperature in anitrogen flow of 20 SCFH with the fan operating at 1500 rpm. Thenitrogen flow is then discontinued and the above-described feed streamis introduced into the reactor. The total gas outlet flow is adjusted to22.6 SCFH. The temperature is adjusted over the next few hours so thatthe ethylene oxide concentration in the outlet gas is approximately1.0%.

3. The outlet oxide concentration is monitored over the next 4-6 days tomake certain that the catalyst has reached its peak steady stateperformance. The temperature is periodically adjusted to achieve 1%outlet oxide. The selectivity of the catalyst to ethylene oxide and thetemperature are thus obtained.

The standard deviation of a single test result reporting catalystefficiency in accordance with the procedure described above is 0.7%efficiency units.

It should be noted that the above-described back mixed autoclavegenerates lower efficiencies than tubular reactors, hence, theefficiencies described herein are not directly comparable with thoseobtained in a tubular reactor. In addition, the catalyst particlestested in the following examples are shaped for use in commercial sizedtubular reactors. Such particles are known to yield lower efficienciesthan crushed catalyst or catalyst made on a crushed support, but theyhave a significant advantage for operation in a commercial reactor inthat they do not create an undesirable pressure drop across the catalystbed as would crushed catalyst or catalyst made on a crushed support.

EXAMPLE 1

A catalyst containing 13 weight % Ag was prepared as hereinafterdescribed on an alpha-alumina carrier "A" shaped as a ring having adiameter of 5/16", a length of 5/16" and 1/8" diameter hole. The carrierhad the following chemical composition and physical properties.

    ______________________________________                                        Chemical Composition of Carrier "A"                                           ______________________________________                                        Alpha-Alumina   98.6 wt. %                                                    Silicon Dioxide 0.74 wt. %                                                    Calcium Oxide   0.22 wt. %                                                    Sodium Oxide    0.16 wt. %                                                    Ferric Oxide    0.14 wt. %                                                    Potassium Oxide 0.03 wt. %                                                    Magnesium Oxide 0.03 wt. %                                                    ______________________________________                                        Physical Properties of Carrier "A"                                            ______________________________________                                        Surface Area.sup.(1)   0.3 m.sup.2 /g                                         Pore Volume.sup.(2)    0.50 cc/g                                              (or water absorption)                                                         Packing Density.sup.(3)                                                                              0.70 g/ml                                              Median Pore Diameter   21 microns                                             ______________________________________                                        Pore Size Distribution, % Total Pore Volume (% TPV).sup.(4)                   Pore Size, Microns                                                                             % TPV                                                        ______________________________________                                        0.1-1.0          1.5                                                           1.0-10.0        38.5                                                         10.0-30.0        20.0                                                          30-100          32.0                                                         >100             8.0                                                          ______________________________________                                         .sup.(1) Method of measurement described in "Adsorption, Surface Area and     Porosity", S. J. Gregg and K. S. W. Sing, Academic Press (1967), pages        316-321.                                                                      .sup.(2) Method of Measurement as described in ASTM C2046                     .sup.(3) Calculated value based on conventional measurement of the weight     of the carrier in a known volume container.                                   .sup.(4) Method of measurement described in "Application of Mercury           Penetration to Materials Analysis", C. Orr Jr., Powder Technology, Vol. 3     pp. 177-123 (1970).   The carrier "A" was impregnated under vacuum as         hereinafter described with a solution of silver salts which was prepared     at a concentration such that the finished catalyst contained the desired     amount of silver. The required concentration of silver in solution for the     given carrier is calculated from the packing density (grams/cc) and the     pore volume of the carrier which are either known or readily determined.     Assuming that all of the silver in the impregnating solution contained in     the pores of carrier "A" is deposited upon the carrier, approximately 23.6     weight % silver in solution is needed to prepare a catalyst containing     about 13 weight % silver.

Preparation Of Silver Impregnating Solution

774.9 gms of ethylenediamine (high purity grade) was mixed with 1600 gof distilled water with continuous stirring in a 7 liter stainless steelbeaker containing a three inch stirring bar, the vessel being mounted ona 6"×6" magnetic stirrer-hot plate. The ingredients were added to thevessel in the order described with constant stirring. The resultingsolution was cooled to 25° C. and 812 g of oxalic acid dihydrate(reagent grade) was added in small portions, with continuous stirring,at a rate which maintained the temperature below 50° C. Silver oxidepowder, 1423.5 g, (Handy and Harmon, 850 Third Avenue, New York, N.Y.10022) was then added intermittently to the aqueous ethylenediamineoxalic acid solution while maintaining the temperature of the solutionbelow 50° C. Finally, 283 g of monoethanolamine and 703 g of distilledwater were added to bring the total volume of the impregnating solutionto 4000 cc. The specific gravity of the resulting solution was about1.385.

Catalyst Preparation

A 2636 grams charge of carrier "A" was placed in a 5 liter, roundbottomed vessel equipped with a side arm fitted with a stopcockconnected to a three foot long, 1/4" O.D. tubing for the introducion ofthe impregnating solution which was contained in the above-described 7liter stainless steel beaker located adjacent to the vessel. The vesselcontaining the carrier was evacuated to approximately 2 inches ofmercury pressure for about 20 minutes after which the impregnatingsolution was slowly added to the carrier by opening the stopcock betweenthe vessel and the beaker containing the impregnating solution until thecarrier was completely immersed in solution. The vessel was then openedto the atmosphere to achieve atmospheric pressure, the carrier remainingimmersed in the impregnating solution at ambient conditions for aboutone hour and thereafter drained of excess solution for about 30 minutes.

The impregnating carrier was removed from the vessel and heat treated asfollows to effect reduction of the silver salt. The impregnated carrierwas spread out in a single layer of pellets on a 25/8" wide endlessstainless steel belt (spiral weave) and transported through a 2"×2"square heating zone for 2.5 minutes, the heating zone being maintainedat 500° C. by passing hot air upward through the belt and about thecatalyst particles at the rate of 266 SCFH. The hot air was generated bypassing it through a 5 ft. long ×2" I.D. stainless steel pipe which wasexternally heated by an electric furnace (Lindberg™ tubular furnace:21/2" I.D., 3 feet long heating zone) capable of delivering 5400 watts.The heated air in the pipe was discharged from a square 2"×2" dischargeport located immediately beneath the moving belt carrying the catalystcarrier. After being roasted in the heating zone, the silver impregnatedcatalyst was weighed, and based upon the weight gain of the carrier, wascalculated to contain 13.1 weight % silver. The silver-containingcatalyst is referred to as catalyst 1.

Addition of Promoters

To demonstrate the effect which the solvent in the alkali metalimpregnating solution has on the efficiency of the finished catalyst,five (5) catalysts of similar composition were prepared (1A-1E) using adifferent impregnating solution for each one, each solution containing adifferent solvent as described below. Catalysts containing 13.1 wt. %silver, 0.00906 wt. % cesium and 0.00268 wt. % potassium were preparedfrom the above described catalyst 1 by the sequential addition of cesiumand potassium promoters in accordance with the following generalprocedure.

Each of the impregnating solutions used to prepare catalysts 1A through1E was prepared by adding (a) 5.825 ml of an aqueous cesium hydroxidesolution containing 0.0566 g of cesium and (b) 4.456 ml of an aqueouspotassium carbonate solution containing 0.0167 g of potassium, to a 250ml graduated cylinder. To each of the graduated cylinders there wasadded one of solvents A through E, identified below, in an amountsufficient to provide 250 ml of total solution.

SOLVENTS

A. Water

B. A solution of 80 wt. % water, 20 wt. % monoethanolamine.

C. A solution of 50 wt. % water, 50 wt. % monoethanolamine.

D. Monoethanolamine.

E. A solution of water-ethylenediamine-oxalic acid-monoethanolamine asdescribed above in the preparation of catalyst 1, but containing nosilver.

For the preparation of each catalyst, a 100 g sample of catalyst 1 wasplaced in a 12" long×1.5" I.D. glass cylindrical vessel equipped with aside arm fitted with a stopcock so as to allow the evacuation of thevessel using a vacuum pump. A 500 ml separatory funnel containing one ofthe impregnating solutions described above was inserted through a rubberstopper in the top of the vessel. The impregnating vessel containingcatalyst 1 was evacuated to approximately 2 inches of mercury pressurefor about 20 minutes after which the impregnating solution was slowlyadded to the carrier by slowly opening the stopcock between theseparatory funnel and the impregnating vessel until catalyst 1 wascompletely immersed. Following the addition of solution, the system wasopened to the atmosphere, catalyst 1 remaining immersed in theimpregnating solution at ambient conditions for about 30 minutes. Theimpregnated carrier was drained of excess solution and heat treated toeffect deposition of alkali metal on the carrier in the same manner asdescribed above with regard to the preparation of catalyst 1. Thefinished catalysts prepared from the impregnating solutions containingone of the solvents A-E are designated 1A, 1B, 1C, 1D and 1E,respectively.

Table I below summarizes the test results for catalysts 1A through 1Ewhen used for the oxidation of ethylene in accordance with the proceduredetailed above.

                  TABLE I                                                         ______________________________________                                        Catalyst   Selectivity, %                                                                            Temperature °C.                                 ______________________________________                                        1A         70.6        261.4                                                  1B         71.8        257.5                                                  1C         73.7        252.4                                                  1D         74.0        256.8                                                  1E         75.3        251.9                                                  ______________________________________                                    

As noted from Table I, catalyst 1A, which was prepared in accordancewith the method of the prior art, provided the lowest selectivity incomparison with catalysts 1B-1E, prepared by the method of theinvention.

What is claimed is:
 1. In the process for preparing a supported silvercatalyst for the production of ethylene oxide by the vapor phaseoxidation of ethylene with an oxygen-containing gas comprising:(a)impregnating a porous catalyst support with a solution comprising asolvent or a solublizing agent, and silver salt in an amount sufficientto deposit the desired amount of silver on said support; (b) treatingthe impregnated support to convert at least a fraction of the silversalt to silver metal and effect deposition of silver on the surface ofsaid support; (c) impregnating the support treated in step (b) with asolution containing a solvent and a compound of at least one metalcation promoter in an amount sufficient to deposit the desired amount ofpromoter on said support; and (d) treating the impregnated supportproduced in step (c) to effect disposition of said promoter on thesurface of said support,the improvement comprising using in step (c) asolution comprising at least 50 wt. % of an organic solvent capable offorming a complex with silver ion and selected from the group consistingof alkyl amines, alkylene diamines, amino alcohols, amino ethers andamides and at least one compound of an alkali metal promoter.
 2. Aprocess as in claim 1 wherein said organic solvent is an amino alcohol.3. A process as in claim 1 wherein said organic solvent is analkylenediamine.
 4. A process as in claim 1 wherein the solution used instep (c) thereof is substantially free of a lower alcohol.
 5. A processas in claim 1 wherein said promoter is an alkali metal selected from thegroup consisting of cesium, potassium, lithium, sodium, rubidium andmixtures thereof.
 6. A process as in claim 5 wherein the amount ofalkali metal deposited on the catalyst support is at least 10% greaterthan that amount of like alkali metal which provides the maximumenhancement of efficiency when used in a coincidental method ofpreparation with the same amount of silver and the same catalystsupport.
 7. A process as in claim 1 wherein said catalyst containssilver in an amount of from about 2% to about 20% based on the totalweight of the catalyst.
 8. A process for preparing a supported silvercatalyst for the production of ethylene oxide by the vapor phaseoxidation of ethylene with an oxygen-containing gas comprising:(a)impregnating a porous catalyst support with a solution comprising asolvent or a solubilizing agent, and silver salt in an amount sufficientto deposit the desired amount of silver on said support; (b) treatingthe impregnated support to convert at least a fraction of the silversalt to silver metal and effect deposition of silver on the surface ofsaid support; (c) impregnating the support treated in step (b) with asolution containing more than 30%, by volume, of at least one liquidorganic solvent capable of forming a complex with silver ion andselected from the group consisting of amino alcohols, alkylene diamine,alkyl amines, amino ethers and amides, and a compound of at least onealkali metal promoter in an amount sufficient to deposit the desiredamount of promoter on said support wherein said solution issubstantially free from a lower alcohol; and (d) treating theimpregnated support produced in step (c) to effect deposition of saidpromoter on the surface of said support.
 9. A process for preparing asupported silver catalyst for the production of ethylene oxide by thevapor phase oxidation of ethylene with an oxygen-containing gascomprising:(a) impregnating a porous catalyst support with a solutioncomprising a solvent or a solubilizing agent, and silver salt in anamount sufficient to deposit the desired amount of silver on saidsupport; (b) treating the impregnated support to convert at least afraction of the silver salt to silver metal and effect deposition ofsilver on the surface of said support; (c) impregnating the supporttreated in step (b) with a solution containing at least about 50%, byvolume, of an organic solvent selected from the group consisting oforganic liquids capable of forming a complex with silver ion, and acompound of at least one alkali metal promoter in an amount sufficientto deposit the desired amount of promoter on said support; and (d)treating the impregnated support produced in step (c) to effectdeposition of said promoter on the surface of said support.